The neutral papermaking can retain paper strength more than acid papermaking, making it possible to extend the time to use felt or wire during papermaking. Hence there are the following advantages that the drainage of web is improved and formation is improved thereby to improve paper quality. Unlike the acid papermaking, the neutral papermaking has fewer problems with paper deterioration and drainage regulation, and is advantageous in terms of water recycling.
Conventionally, the sizing agent composed of alkyl ketene dimer has been known as the sizing agent used in the neutral papermaking. However, the alkyl ketene dimer-based sizing agent has the disadvantage that the sizing effect immediately after papermaking is poor due to a slow rise of the sizing effect. Additionally, there has been the problem that the alkyl ketene dimer-based sizing agent is generally expensive and thus increases costs. Although the neutral rosin sizing agent using rosin ester or reinforced rosin ester has also been known, the neutral rosin sizing agent has the problem that it is difficult to obtain the initial degree of sizing unless papermaking conditions are properly controlled when using the sizing agent. The neutral rosin sizing agent also has the problem that due to its low sizing performance, there is need to increase the addition rate of the sizing agent, thus increasing sizing costs.
Heretofore, as a sizing agent usable in the neutral papermaking, the sizing agent composed of the quaternized form of hydrophobic-group-containing cationic polymer obtained by quaternizing a copolymer composed of a styrene homologue and aminoalkyl ester of (meth)acrylic acid with alkyl halide has been proposed (refer to patent document 1). Other sizing agent, which is adapted to improve not only sizing effect but also the strength and the friction coefficient of paper by optimizing the kind of the quaternizing agent and the kind of the cationic monomer used in the above sizing agent, is also known. Specifically, there has been proposed the sizing agent composed of the quaternized form of hydrophobic-group-containing cationic polymer obtained by quaternizing the above copolymer with epihalohydrin instead of alkyl halide (refer to patent document 2), and the sizing agent composed of a copolymer whose constituting monomer is styrenes and aminoalkyl ester of (meth)acrylic acid, amino acrylamide of (meth)acrylic acid, or quaternary salts thereof (refer to patent document 3). These sizing agents are cationic and hence self-fixed onto anionic chargeable pulp fibers thereby to impart sizing performance to papers without using any fixing agent such as aluminum sulfate, thereby enabling neutral papermaking or alkaline papermaking.
With the aim of further improving sizing effect, there has been proposed the sizing agent composed of rosin-bonding cationic polymer obtained by allowing a predetermined amount of rosin derivative to coexist during the copolymerization of a hydrophobic monomer and a cationic monomer (refer to patent document 4). This sizing agent enables the bulky structure of the rosin incorporated into polymer to impart high hydrophobicity after the fixing onto pulp fibers. Additionally, owing to the interaction between the carboxyl groups of a rosin ring and calcium carbonate, the polymer itself becomes insoluble, and the fixing through cationic groups is further facilitated, thereby permitting a considerable improvement in sizing effect.    Patent document 1: U.S. Pat. No. 2,964,445    Patent document 2: Japanese Unexamined Patent Application Publication No. 48-11407    Patent document 3: Japanese Unexamined Patent Application Publication No. 3-167397    Patent document 4: Japanese Unexamined Patent Application Publication No. 2001-73292